Combustion misfires of internal combustion engines may result in increases of toxic substances in exhaust gasses that may cause air pollution and deterioration in engine performance. It is therefore necessary to determine whether a combustion misfire occurs and to take suitable action for the combustion misfire as the engine continues to operate. A conventional method exists for determining a combustion misfire based on engine rpm change. For example, U.S. Pat. Nos. 5,875,411 and 5,861,553 disclose methods for determining a combustion misfire on the basis of an engine rough-running value.
However, the conventional method involves calculation of an engine rough-running value through digital filtering of a segment time, so the calculation process and the calculation load increase substantially.